Rare earth separation by anion exchange



April 1953 E. H. HUFFMAN ETAL 2,635,044

RARE, EARTH SEPARATION BY ANION EXCHANGE Filed Jun'e 22. 1951 Volume of Eluiriont O.IOOM OITRIC ACID Volume of Elutricm? O.IOOM GITRIO ACID xlo N r m m5 u mfl V m T R E 5 W0 R Y B m m 5 6 o 6 5 5 0 5 5 4 o 4 5 3 o 3 Volume of Eluirluni A T TOR/V5 Y.

0.0125! GITRIC ACID Patented Apr. 21, 1953 E'FECE RARE EARTH SEPARATION BY ANION EXCHANGE Application June 22, 1951, Serial No. 233,088

Claims. 1

This invention relates to a process for the separation of rare earth elements and, more particularly, to an ion exchange process employing rare earth-citrate complex anions for separating rare earth elements from each other.

For many years the separation of rare earth or lanthanide elements has presented great process difliculties and especially the separation of individual members of the group. These difficulties stem from the fact that the chemical and physical properties of the rare earths are so similar that any single operation (fractional crystallization, extraction, etc.) must be repeated an enormous number of times to achieve even a partial separation. Recently, practical methods of separating rare earth elements have been developed using cationic exchange resins. These methods, in essence, automatically repeat a single operation (distribution of a cationic rare earth species between an aqueous phase and a solid resin phase) many times and thus are able to magnify the small distribution differences between each rare earth to such an extent that very complete separations are possible with a minimum expenditure of time and labor.

We have now discovered that under certain specific conditions anionic rare earth species may be formed and that these anionic species may be employed in conjunction with certain anionic exchange resins to eifect a very simple and efficient separation of rare earth elements from each other.

It is therefore a principal object of the present invention to provide a simple, practical, and efficient method of separating the rare earth elements from each other.

Another object of the invention is to provide aqueous citrate solutions containing anionic species of rare earth elements for use in anionic separation processes.

A further object of the invention is to utilize the exchange of rare earth-citrate complexes between an anionic exchange resin and a solution to provide a separation of rare earth elements.

Other objects and advantages will become apparent from the following description considered together with the attached drawing, in which:

Figure l is a graph illustrating the elution of promethium and europium with 0.100 M citric acid elutriant at a pH of 1.70;

Fig. 2 is a graph illustrating the elution of promethium and europium with 0.100 M citric acid elutriant at a pH of 2.04; and

Fig. 3 is a graph illustrating the separation of promethium and europium obtained by elution izyilti a 0.125 M citric acid solution at a pH of Ion exchange resins constitute essentially an insoluble hydrocarbon matrix bearing a large number of active groups capable of exchanging. the ionizable components thereof for other ions of the same sign. Anionic exchange resins of the "strong base type, which are particularly suited for use in the process of the present invention, possess active groups such as quaternary ammonium, substituted guanidinium, etc., in which the ionizable component (e. g., 01-, OH) is capable of being replaced by other anionic species. Strong. base type anionic exchange resins, such as those mentioned above, are characterized by showing no hydrolytic efiects when placed in aqueous solu-- tions.

As an example of an anion exchange reaction,- one may consider the treatment of a strong base type resin (quaternary ammonium active groups) in the chloride form (i. e., 01'" is the replaceable anion) with an aqueous nitrate solution. The reaction which ensues may be represented by the following equation:

[R4N]+C1 NO3 [R4N]+NO3 01- In this equation, the first and third members represent respectively the chloride and nitrate forms of the resin and have been written to illus-: trate the ionic bonding between the resins active groups [R4N]+ and the replaceable anion. This" type of bonding is an inherent characteristic of strong base type resin as differentiated from other The net result of the above ion exchange resins. reaction is that nitrate ions have been removed from solution and adsorbed on the resin by undergoing ion exchange with the resins replaceable" chloride ions. As the completeness of anion exchange reactions is dictated by conventional equilibrium expressions, such reactions may be driven essentially to completion by using an excess of an appropriate reactant.

In the process of the present invention, a portion of finely-divided strong base type anionic plexes are then eluted from the resin column with an elutriant consisting of a citric acid solution of specified pH.

great, each rare earth complex will accumulate in a sharply-defined region or band,which slow=-' tion phase will preceed those that contain rare:

earth species favoring the resin phase. We have found that under certain critical experimental conditions that negligible overlapping ofnthe in dividual bands may be achieved. Thus by separatelycollecting those fractions of the elutriant issuirrgffromthe resin column and--conta-ining thefiindividual" bands; highly purified solutionsofeach rare earth may be 'obtainedl The two crite'riafbr the s'ujccessful separation of rare earth element's bythe present invention are the formation of anionic rare earth-citrate complexes and the elution of such complexes from; an 'aliiohic exchangefresm with" little overlappingof the individual rare earth comp1ex band's: While appreciable amounts of anionic rar" earth-citrate com lexes can be formed-by g soluble triva ent rareeartn salts in "(1* citric acid solutions of a wide variety of compositions; we have foun'd" thatoperable" solution compositions are quite definitely limited tof therelatively-narrow range of about 1.5"- to 3.0 pI-f' valuesi While the foregoing range of pH valuesmay' be considered the operable range for the whole rare earth series; We" have found further that the pH and concentrationof'the citric acid' solution will be within narrower'limits in certain cases withparticulancombinations oi rareearths in the original material. For example, with a citrici'acid concentration of' 0.01'25 molar and? a oiriapproximat'e'ly 2.1;" (as determined: by a glass electrode) rare earths such-t as r europium: promethium are? most advantageously sepa-' rated.

Imthe: practice of the present invention-it has beers-found desirableeto employ citrate solutions" of: the some; composition for each step v of-- the,- Iiw esssifor the conversion of the; resin to: the; citrate form in: the; preparation of the rare:- eairthscitratew complex solution,. and as the elutriant. Radioactive tracer quantities ofthe rare earths have been employed todeterminetheoptimum; processconditions within: the highly criticalpH range because of theeaseof analyzingthe composition of the elutriant-issuing fromthe ionexchange column by radioactive assay. However, batch equilibration testshave demonstrated -that-= the-capacityot the resin is' su-fficient'to permit large scale rare earth-separations:

The following example demonstrating the-- separation-of promethium-from europiumfurther illustrates the processof our. invention and its utility in providing individual rare earth values in hig-h purity.-

' Exampleacidwas adsorbed on" theup'pert; region-"or a column of 250-500 mesh Dowex A-l resin (a quaternary ammonium type resin) 14.9 cm. long and 0.08 cm? cross section. This resin had been previously converted to the citrate form by treating the original chloride formoi the resin with a citrate solution of a similar concentration and pH as that given above. The resin column was then eluted at a rate of 1.5 ml./hr. with a citrate solution of similar composition. Radioactive analysis-of appropriate fractions of the elutriant leaving the column yielded the values plotted in the curves of Fig. 3. It is apparent from Fig. 3, th'anan 'excellentsep'aration of these rare earths was achieved; It may be noted that the elution is in order of increasing atomic number. This order of elution is the reverse of that obtained by cation exchange methods and is a most advantageous feature as in certain cases a desired rare earth may be eluted first whereas in the cationic process the same rare earth is eluted last:

The; e1ution"curves"shown in 1 and- 2 definitely illustrate the'efiects' of citric acid com centration andpH on'the elution behavior of rare, earths'in the process of the present inve'ri tion. These curves: were obtained 'under'experimental conditions identical to those set forth in the foregoing xample; except that theamounts or tracerPrr'i and Eu were 20,000 counts/ran ute each and the composition and pH of the citric acid solutionwerethos'e' iridicatedthereifi'. From a comparison of Figs. 1', 2, and 3; it is ap': parent that lowering'the pH below the optimum of 2110 moves the individual rare earth peaks to-" g'etherand that raising the molarity'of the citric: acid tends to flatten or s'm'ear thindividual? peaks;

While the-salient 'features" of this iriven'ticiri havebeen described in detail to one embodiment it will, of course, be' apparent" that-numerous" modifications maybe made within. thespirit and" scope of" this invention, and it is not therefore" desired to limit the invention to theexact' details described except insofar as they may be defined: in the following claims.

What isclair'nedis: v

1".- In' amethod for separating rare earth ele ments from each other, the"steps'"comprising ad'- sorbing 'rare" earth-citrate complexes; from" an acidic aqueous citrate" solution on a strong base" type anionexchange resinposs'essing exchangeable" citrate anions, and selectively" eluting. each adsorbed rare earth? complex in the order of increasing rare earthatomi'c number with a citric acid solution having a pH value 'inthe' range. of"

about'1.5 to 3.0.

' 2. The method of preparing anionic resin. a'd sorbable anionic rare earth-citrate complexesih solution which comprises. dissolving solubletrivalent rare" earth salts in a solution having a forming" rare earth citrate complex anions solution by dissolving trivalent rare earth. salts in'-an aqueous "citric acid solution having'a citric acid concentration in the range 0005-01150. molar and acidified to a pH value in'the range of 1.50 3.0, contactingijan anionicf exchange resin in the citrate form with said complex-contaihingsolution whereby the anionic rare; earth-citrate are adsorbed onthe resin", selectively elutingthe adsorbedrare earth complexes with a solutionhav in'ga)" 03005 to 0150' molar concentration of citric 5 acid and a pH of 1.5 to 3.0, and collecting the individual fractions of the elutriant containing the separated rare earths.

4. The process as defined in claim 3 wherein said rare earths comprise europium and promethium.

5. The process as defined in claim 3 wherein said rare earths comprise europium and promethium, and the elutriant solution comprises 0.125 molar citric acid and a, pH of about 2.1.

EUGENE H. HUFFMAN. ROBERT L. OSWALT.

References Cited in the file of this patent Tomkins et a1.: Jour. Am. Chem. Soc., vol. 69, p. 2769-2770.

Krause et a1.: Jour. Am Chem. Soc., vol. 71, pp. 3263 and 3855 (1949).

Hufiman et a1.: Jour. Am. Chem. Soc. vol. 71, p. 4147. 

1. IN A METHOD FOR SEPARATING RARE EARTH ELEMENTS FROM EACH OTHER, THE STEPS COMPRISING ADSORBING RARE EARTH-CITRATE COMPLEXES FROM AN ACIDIC AQUEOUS CITRATE SOLUTION ON A STRONG BASE TYPE ANION EXCHANGE RESIN POSSESSING EXCHANGEABLE CITRATE ANIONS, AND SELECTIVELY ELUTING EACH ADSORBED RARE EARTH COMPLEX IN THE ORDER OF INCREASING RARE ATOMIC NUMBER WITH A CITRIC ACID SOLUTION HAVING A PH VALUE IN THE RANGE OF ABOUT 1.5 TO 3.0. 